Conventionally, there has been known a control apparatus for an internal combustion engine which calculates a combustion ratio MFB (Mass Fraction Burned or Burnt) on the basis of a cylinder pressure (pressure within a combustion chamber) detected by cylinder pressure detection means, and controls ignition timing (combustion start time) such that the combustion ratio MFB at a predetermined crank angle coincides with a target combustion ratio. Such an apparatus is configured to control the ignition timing SA such that the combustion ratio MFB8 at a crank angle 8° after compression TDC becomes 50%. Thus, even in the case where individual differences are present among internal combustion engines, appropriate ignition timing is provided to each engine. Accordingly, combustion efficiency is improved, and each internal combustion engine can output an increased torque (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-144645).
The combustion ratio MFB is substantially equal to a ratio of indicated heat quantity. The ratio of indicated heat quantity is defined as a ratio Qsum/Qtotal in a single combustion stroke, where Qtotal is the total quantity of heat which is a portion of heat generated by all fuel burnt in a combustion chamber and which was converted to work for a piston, and Qsum is a cumulative quantity of heat which is a portion of heat generated by fuel burnt in the combustion chamber until a predetermined timing and which was converted to work for the piston. The combustion ratio MFB is defined as a ratio of a cumulative quantity of fuel which is a portion of fuel burnt in the combustion chamber until the predetermined timing and which contributed to work for the piston, to the total quantity of fuel which is a portion of all fuel burnt in the combustion chamber and which contributed to work for the piston.
Meanwhile, there is widely known an internal combustion engine which includes a variable valve timing apparatus (VVT) which can control the open and close timings of intake valves and/or exhaust valves. One of such variable valve timing apparatuses controls a period in which both the intake valves and the exhaust valves are maintained open (hereinafter referred to as the “valve overlap period” or “overlap period”) by advancing or delaying the open timing of the intake valves.
In general, prolongation of the overlap period results in an increase in the quantity of a burnt gas which is discharged from a combustion cumber to an intake port and is again taken into the combustion cumber (the burnt gas is also called “internal EGR gas” or “self EGR gas”). In other words, the variable valve timing apparatus functions as a burnt gas quantity control apparatus.
When the burnt gas quantity is increased by means of a burnt gas quantity control apparatus such as the variable valve timing apparatus or an external EGR apparatus, combustion speed decreases. That is, as shown in FIG. 15, the quantity of heat which is a portion of heat generated by fuel burnt in a combustion chamber and which contributed to work for a piston (indicated generated heat quantity) increases at a slower rate when the burnt gas quantity is relatively large (the inclination of a curve indicated by a solid line of FIG. 15), as compared with the case where the burnt gas quantity is relatively small (the inclination of a curve indicated by a broken line of FIG. 15). That is, as the burnt gas quantity increases, combustion speed decreases. As a result, combustion temperature lowers, whereby the quantity of discharged NOX decreases.
Moreover, when the burnt gas quantity increases, combustion progresses more gradually, whereby the combustion period is prolonged. Accordingly, a greater quantity of fuel can be burnt. That is, a period T1 of FIG. 15 becomes longer than a period T2, so that a total heat quantity Qtotal1 corresponding to the total quantity of burnt fuel becomes greater than a total heat quantity Qtotal2. As a result, as the burnt gas quantity increases, the quantities of discharged HC and CO become smaller. However, when the burnt gas quantity is excessive, combustion becomes instable. As a result, the quantities of HC and CO increase.